Tritiated water, or super-heavy water, can be denoted as HTO or T.sub.2 O, where T refers to tritiumn, an elemental hydrogen isotope having three neutrons in its nucleus. The compound T.sub.2 O is unstable in water, and generally forms HTO. Tritium decays by emitting .beta. particles (electrons), a form of radiation which is potentially harmful due to its ability to ionize living matter.
Tritiated water is a by-product of nuclear power generation, being produced in the primary heat transport systems of certain types of nuclear reactors, such as CANDU reactors. In such reactors, heat is transferred from the primary heat transport system to a secondary heat transport system, both of which are closed systems, via a heat exchanger. The heat transferred from the primary system to the secondary system is used to generate steam in the secondary system, and excess heat is then removed from the water in the secondary system by service water brought in from a body of water located adjacent to the power station. The heated process water is then returned to the body of water.
Occasionally, tritiated water leaks from the primary to the secondary heat transport system. It is preferred that such leaks are quickly detected and corrected in order to minimize heat loss and also to prevent tritium contamination of the water in the secondary system, thereby minimizing the likelihood that tritiated water could be released into the environment in the discharged service water. Therefore, tritium levels in the secondary heat transport systems of nuclear plants are routinely monitored to detect leakage of tritiated water from the primary to the secondary heat transport systems.
Currently, the most effective method for monitoring tritium levels in waste water is liquid scintillation counting (LSC). The detection limit of LSC is quite low, of the order of tens of nCi/L which can potentially allow detection of a leak from the primary to the secondary heat transport systems in CANDU reactors of the order of 0.1 L/h. In LSC, a sample of potentially tritiated water is mixed with a liquid scintillant and the mixture is then monitored for photoactivity with the aid of one or more photomultipliers. Specifically, the liquid scintillant effectively surrounds each water molecule such that a .beta. particle emitted by a tritiated water molecule excites the scintillant, causing the scintillant to emit a photon which is detected by the photomultipliers.
LSC monitoring is frequently referred to as "grab sampling", with a typical nuclear power station collecting and analyzing up to 40 to 50 sample vials each day from the secondary heat transport system, which is quite labour intensive and does not permit leaks to be detected quickly. An automated version of LSC is known which monitors tritium-in-water levels on a semi-continuous basis. One major disadvantage of LSC is that particulate and biological matter removed from the water sample by filtration frequently cause fouling of the filter element, resulting in high maintenance costs. Further, the organic scintillant combined with the tritiated water sample in LSC cannot be returned to the environment, and therefore the LSC samples must be disposed of as low level radioactive waste.
A less common method for tritium monitoring is solid scintillation counting (SSC). In SSC, a sample of water is passed between two closely spaced sheets of plastic containing a solid scintillant. The .beta. particles produced by the decay of tritium atoms in a thin layer of water immediately adjacent each sheet are deposited on the solid scintillant, which in turn emits a photon to be detected by a photomultiplier. SSC is fundamentally less efficient than LSC because it relies on surface detection while LSC is based on volume detection. Automated SSC detector systems have demonstrated detection limits around 1 .mu.Ci/L.
SSC also requires filtration and is therefore subject to the same disadvantage as LSC in regard to filter fouling. In addition, SSC is subject to memory effects from tritium retention on the solid scintillant, thereby reducing its effectiveness.
Clearly, a more effective method for monitoring tritium levels in water is needed.
Some of the above disadvantages of current methods for monitoring tritium levels in water may partially be overcome by the method disclosed in U.S. Pat. No. 3,489,903, issued Jan. 13, 1970 to Robinson. This patent describes a method of measuring tritium levels in a sample of urine in which a known volume of tritium-contaminated urine is vaporized in a confined zone such as an ionization chamber. The .beta. particles produced by tritium decay traverse the ion chamber, producing a number of ionizations which are measured as a current signal in the ionization chamber. One of the primary advantages of this method is that it allows rapid measurement of tritium levels in a liquid sample.
The apparatus described by the Robinson patent is adapted to determine whether or not the tritium level in the urine exceeds a maximum acceptable tolerance of 50 .mu.Ci/L. To the inventor's knowledge, it has not been adapted to use in detecting leaks of tritiated water from nuclear reactors. In any event, the sensitivity of the Robinson method would likely not be acceptable for leak detection in a nuclear power plant, since the detection limit of 50 .mu.Ci/L disclosed by Robinson translates to a leak of about 100 L/h from the primary to the secondary heat transport systems in a CANDU reactor, which is substantial.
It would be desirable to provide a simple and effective method for tritium-in-water monitoring which is capable of quickly detecting low levels of tritium in a sample of water to thereby provide fast and effective detection of leaks in a nuclear reactor.